Patent GB-929842-A (GB)
Title: Gas turbine engine fuel system
Inventor: JOHNSON CHRISTOPHER LINLEY
IPC: F02C7/22
Filed: 1960-12-14
Abstract: 929,842. Gas-turbine engine fuel systems. ROLLS-ROYCE Ltd. Dec. 7, 1961 [Dec. 14. 1960], No. 43119/60. Class 110 (3). [Also in Group XXIX] A gas-turbine engine fuel system comprises a constant displacement pump, means for providing the low pressure side of the pump with fuel, a high-pressure conduit communicating with the high pressure side of the pump, a fuel passage adapted to be connected to the engine burners so as to supply the latter with fuel, a spill passage leading to the low pressure side of the pump metering valve means, the position of which controls the relative flow of fuel from the high pressure conduit to the fuel passage and to the spill passage, and means for adjusting the position of the metering valve means and so of the flow through the fuel  passage, in dependence on the value of one or more engine variables. The fuel system of a gas turbine engine such as a vertical lift engine is shown and comprises a fuel tank 10 from which fuel is supplied by means of a booster pump 11 through a conduit 12 to the inlet of a constant displacement, engine driven pump 13 which may be a gear-type pump, the pump delivering fuel at high pressure into a conduit 14 in a control unit 15. The fuel passes through a restriction 16 and duct 17 to a chamber 18 in which is disposed a metering valve 24, the valve comprising a sleeve formed with orifices 40 and 42, the orifices 40 co-operating with an annular chamber 41 which communicates with a duct 44, while the orifices 42 co-operate with an annular chamber 43 which communicates with a spill passage 45. The metering valve is connected by a shaft 29 to a sleeve 30, the sleeve at its left-hand face communicating with a chamber 37 which is connected by way of a duct 38 with high pressure fuel in the duct 14. The pressure at the right-hand side of the metering valve is the pressure down-stream of the restriction 16 so that the metering valve takes up a position dependent on the pressure drop across the restriction. This pressure drop is proportional to the square of the engine speed N. A needle valve 19 may be associated with the restriction 16 as shown, the needle valve being positioned by means of a bellows 20 in dependence on the compressor intake temperature T 1 . The pressure drop across the restriction 16 is then proportional to N<SP>2</SP>/T 1 . The metering valve 24 is loaded towards the left by a spring 28 while the sleeve 30 is loaded towards the right by an adjustable spring 33. The metering valve is rotated continuously during operation by means of a pinion 63 which engages a gear 64 formed on the valve. The duct 44 communicates with a duct 72, also with a chamber 46, the duct 72 communicating with a throttle and pressurizing valve 73 which in turn communicates with a main burner 74, a starting jet 75, a dump passage 76 and a spill passage 78. The spill passage 45 communicates with a chamber 47. A spill pressure control valve 48 is disposed between the two chambers 47, 46, the valve being formed with orifices 51 which co-operate with the partition 49 so as to constitute a variable area orifice, fuel from the spill passage 45 and chamber 47 passing through the orifice 51 into the chamber 26 from which it passes through duct 27 back to the suction side of the pump 13. The spill pressure control valve 48 serves to maintain the pressures in the passages 44, 45 equal to each other. A flow adjustment valve 57 may also be provided, the valve being formed with a recess 60, the depth of which increases towards the right. The axial position of the valve is controlled by a bellows 66 in dependence on compressor inlet pressure P 1 . The recess 60 controls communication between a passage 53, which communicates with the chamber 18, and a duct 55 which communicates with the chamber 47. The sleeve 48 and the valve 57 are both rotated continuously during operation from the pinion 63. A governor 80 may also be provided to spill fuel into the chamber 26 and back to the inlet of the pump 13 when the engine speed exceeds a selected valve. In operation, as engine speed increases, so also does the speed of the pump 13 and pressure in the duct 14, the pressure drop across the restriction 16 rises and so the metering valve 24 moves to the right. This results in increased fuel flow through ducts 44 and 72 and in a reduction in fuel flow to the spill passage 45 and so to the low pressure side of the pump. A decrease in the value of T 1  will also increase the pressure drop across the restriction 16 and so produce similar results. Should the pressure in the inlet pipe 12 to the pump 13 fall, then the pressure in the duct. 27 and chamber 26 will fall correspondingly The delivery pressure will remain substantially constant so that the spill flow through the duct 45 and ports 51 will increase. The pressure in chamber 47 will also be reduced due to the fall of pressure in chamber 26 and so the spill pressure control valve 48 will move to the left and so reduce the effective size of the ports 51 until the pressures in the chambers 46, 47 are equalized. The same amount of fuel is therefore by-passed back to the pump 13 and the quantity of fuel supplied to the burners 74 is not therefore substantially affected by fluctuations in the pressure in the pipe 12.